Electrode for analytical voltammetry

ABSTRACT

Liquid mercury and liquid diluted mercury amalgams have been the major electrode systems employed in voltammetry and related methods. This is mainly due to their high overvoltage to hydrogen, which enables the determination of heavy metals (zinc, nickel and cobalt etc.) and other species with high negative half wave potentials. The toxicity of mercury and liquid diluted mercury leads to ever increasing restrictions in the their use. The use of such system may even be forbidden in the future at least in online systems for work in the field. Recent work, carried out in our laboratory, has demonstrated that a non-toxic solid dental amalgam may be used as the electrode material, conveniently replacing mercury. An extension of this work has shown that electrode materials comprising a metal or a compound with low hydrogen overvoltage change their hydrogen overvoltage properties substantially when contaminated with even small amounts of metals or compounds which show high hydrogen overvoltage. This extends greatly the range of potentially available electrode systems and thereby analytical possibilities of voltammetry.

INTRODUCTION

[0001] The present invention relates to an electrode for use inelectrochemical analysis and electrochemical processes, especially inanalytical voltammetry.

[0002] Liquid mercury and liquid diluted mercury amalgams have been themajor electrode systems employed in voltammetry and related methods.This is mainly due to their high overvoltage to hydrogen, which enablesthe determination of heavy metals (zinc, nickel and cobalt etc.) andother species with high negative half wave potentials.

[0003] Mercury has been, since the development of polarography, the mostused electrode material in voltammetric techniques. The main reason forthis is that a new fresh electrode surface is formed for each mercurydroplet. Mercury has however, the added advantages that it has a highovervoltage towards hydrogen and that the formation of diluted amalgamsprevents the formation of intermetallic compounds in many cases.

[0004] Due to the toxicity of mercury, its use is now restricted and maybe prohibited, even for analytical purposes. It is necessary therefore,to develop new electrode materials to meet the need for a non-toxicelectrode material in voltammetry.

[0005] Numerous papers dealing with alternative electrodes andtechniques have been published recently. Among these the glassy carbonelectrode², graphite electrode³⁻⁵, gold electrode⁶, silverelectrode^(7,8), and bismuth electrode⁹ are important contributions.But, except from the bismuth electrode, there is a main drawback forthese types of electrodes; their use is limited below −800 mV,restricted by the lack of hydrogen overvoltage.

[0006] The present authors have recently invented a method to substitutemercury with dental amalgam and related solid compounds¹⁰ in electrodesystems. It has been found that such systems have a very highovervoltage to hydrogen, allowing for trace analyses at potentialssufficiently negative to allow determination of e.g. zinc, cobalt,nickel and iron at trace levels. This has not previously been possibleexcept through use of mercury, or mercury film electrodes. Suchdeterminations are very important for field and online analyses ofpollutants in soil and ground-water, and the electrode may be usedrepeatedly.

[0007] The phenomena governing hydrogen overvoltage have theirfoundation in those mechanisms by which the hydrogen evolution reactionoccurs for a given metal. Important factors are the steps in theevolution of hydrogen, the energy of the metal-hydrogen bond, thedependence of the surface coverage by hydrogen on the over potential,the double-layer structure, and the pre-exponential factors of thekinetic equation for the slow stage of the process¹¹. The way by whichthe electrode material or the electrode-surface affects these factors isnot properly known. Some important trends are however identified:

[0008] For metals such as lead, mercury, thallium, cadmium, bismuth etc,which possess high hydrogen overvoltage, an increase in the exchangecurrent density is found with increased hydrogen adsorption energy. Thismeans that the current density at the equilibrium potential increasesfor these metals, with increased strength of the bond of hydrogen tometal. Accordingly a low coverage with hydrogen is found at theirsurface and the rate-determining step must be the proton dischargereaction.

[0009] For metals such as platinum, rhenium, rhodium, tungsten, iridium,molybdenum, etc. high values of hydrogen-adsorption energy is observedand a decrease in the exchange current density is found with an increasein the hydrogen adsorption energy, and high hydrogen coverage¹².

SUMMARY OF THE INVENTION

[0010] The present invention has been conceived to provide a solution tothe problems with prior art electrodes as described above.

[0011] In accordance with a first aspect of the invention there isprovided an electrode for use in electrochemical analysis orelectrochemical processes. The electrode comprises an alloy in a solidstate, the alloy comprising a pure metal or compound having a lowovervoltage for hydrogen and a few percent of at least one second metalor compound, obtaining an electrode with a sufficiently high overvoltagefor hydrogen allowing detection of a metal or compound to be detected.

[0012] In accordance with a second aspect of the invention there isprovided a means for performing electrochemical analyses orelectrochemical processes involving a redox reaction at an electrodesurface, comprising an analysis cell, a system of electrodes arranged inthe analysis cell filled with a solution to be analysed producing ameasuring signal as a consequence of a redox reaction at the electrodes,wherein the measuring signal is a measure of the concentration of acomponent in the solution. At least one of the electrodes comprises analloy in a solid state, the alloy comprising a pure metal or compoundhaving a low overpotential for hydrogen and a few percent of at leastone second metal or compound, obtaining an electrode with a sufficientlyhigh overpotential for hydrogen allowing detection of a metal orcompound to be detected.

[0013] In accordance with a third aspect of the invention there isprovided a method for increasing the utility of voltammetric analyses.The method comprising using an electrode system comprising at least oneelectrode of alloy in a solid state, the alloy comprising a pure metalor compound having a low overvoltage for hydrogen and a few percent ofat least one second metal or compound, obtaining an electrode with asufficiently high overvoltage for hydrogen allowing detection of a metalor compound to be detected.

[0014] The pure metal or compound may be metallic silver, gold, copperor platinum or another metal or compound with too low overvoltage forhydrogen to be used alone for a certain analytical purpose. Mercury,bismuth or lead oxide or another metal or compound with the effect ofincreasing the resulting overvoltage for hydrogen, may be used as thesecond metal or compound.

[0015] In a preferred embodiment the alloy comprises less than 10% ofthe second metal or compound. In an even more preferred embodiment ofthe invention the alloy comprises about 4% of the second metal orcompound.

[0016] The electrode may be used as a measuring electrode or any otherelectrodes in an electroanalytical method, requiring high overvoltagefor hydrogen, as a measuring electrode in voltammetric analysis,preferably of the type differential pulse anodic stripping voltammetry.

[0017] Electrode materials with a metal or a compound with low hydrogenovervoltage change their hydrogen overvoltage properties substantiallywhen contaminated with even small amounts of metals or compoundspossessing high hydrogen overvoltage. This extends greatly the range ofpotentially available electrode systems and thereby analyticalpossibilities of voltammetry.

[0018] The voltammetric behaviour of som non-toxic electrodes that havebeen contaminated with small amounts of metals or compounds possessinghigh hydrogen overvoltage was inv stigated. It was found that theseelectrodes increase their hydrogen overvoltage dramatically. This means,for instance, that the nobl metals may be given a high hydrogenovervoltage. This allows for n w opportunities in voltammetry andrelated methods. Not only as a large number of different undoubtedlynon-toxic electrode metals and electrode compounds may be available, butsuch systems also allow for the development of customise electrodes forspecific analyses.

[0019] The counter electrode may also be made from a composite compoundin order to act as a reservoir for the re-plating of the electrodesurface. In this way a new electrode surface may be generated prior toeach scan. Such results have also been demonstrated in previous workusing pure silver electrodes¹³.

[0020] The invention is stated in the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0021] Embodiments of the invention will now be described in thefollowing with reference to the accompanying drawings, where

[0022]FIG. 1 shows a cross section of a metal alloy electrode accordingto an embodiment of the invention,

[0023]FIG. 2 is a plot of a sweep from −1200 mV to −200 mV with a silverelectrode contaminated with 4% bismuth according to an embodiment of theinvention,

[0024]FIG. 3 is a plot showing detection of zinc on the electrode inFIG. 2,

[0025]FIG. 4 is a plot showing detection of zinc on a silver electrodecontaminated with 4% lead(II) oxide according to an embodiment of theinvention,

[0026]FIG. 5 is a plot showing detection of zinc using a silverelectrode contaminated with 4% mercury according to an embodiment of theinvention, and

[0027]FIG. 6 is a plot showing simultaneous detection of zinc andcadmium on the electrode in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION EXPERIMENTAL

[0028] Three distinct silver-based lectrodes were produced in connectionwith this preliminary paper. These were contaminated with four percentbismuth, four percent lead oxide and four percent m rcury, respectively(all by w ight). The lectrodes were prepared by simply mixing powderedsamples of the given in a mixing capsule. The capsule was subsequentlyinstalled in a mixer and mixed for 30 seconds. The alloy was thantransferred to a quartz tube (internal diameter 4 mm) which was thenevacuated and sealed. The sealed quartz tube was then transferred to anoven preheated to 1000° C., where the metal alloy was melted and keptfor 2 min. The alloy was thereafter allowed to cool down and solidify.The quartz tube was subsequently crushed, and the final metal alloycylinder retrieved and sealed inside an inert plastic membrane in such away that only a small surface was exposed. Finally, the electrode endwas cleanly cut and polished once using a fine soften sandpaper, andwashed in water purified by Millipore Elix and then with MilliporeMilli-Q Gradient system (Millipore Corporation, SA 67120 MolsheimFrance). The electrode is shown in FIG. 1.

[0029] All experiments were performed in NH₄Ac (0.05 M, pH 6). Athree-electrode system where the potential was measured vs. asilver/silver chloride/saturated silver chloride/saturated potassiumchloride reference electrode was used. The counter electrode in thesepreliminary tests was a platinum wire, however a counter electrode withsame alloy composition as the working electrode may also be used.

[0030] The custom voltammetric equipment was constructed in ourlaboratories. This comprises a digital device capable of performing allmodes of voltammetry. This equipment has been described previously¹⁴.All reagents were of analytical grade.

[0031] The working alloy electrodes were polished prior to theexperiments and then used without any maintenance during the experimentover a period of ca one week.

[0032] All of the analyses reported in this preliminary paper wereperformed as differential pulse anodic stripping voltammetry (DPASV),with a scan rate of 15 mV/S, pulse height 70 mV and a deposition time oftwo minutes.

[0033] 3. Results

[0034] This section is divided into three parts. The first part showsresults obtained using an electrode with an alloy composed of silver(96%) and bismuth (4%). The second part summarises results from amixture of silver (96%) and lead(II) oxide (4%), and the third partgives the result from an alloy containing silv r (96%) and mercury (4%).

[0035] 3-1 Silver Lectrode with 4% Bismuth

[0036] The voltammetric scan was performed as described in section 2.Both deposition and start potential was −1200 mV. FIG. 2 shows a typicalplot of a blank NH₄Ac (0.05 M) solution. In FIG. 2 a sweep is performedfrom −1200 mV to −200 mV with a silver electrode contaminated with 4%bismuth. Insert as a small picture up in the left corner a typical plotof a pure silver electrode. The scan was performed in DPASV mode inNH₄Ac (0.05 M), with a scan rate of 15 mV/s, and pulse height 70 mV.

[0037] A pure silver electrode may normally be used down to about −750mV, before the current increases dramatically due to the hydrogenformation, as shown in the small frame up in the left corner in FIG. 2.There is no doubt that the addition of small amount of bismuth gives thesilver alloy electrode a high over potential. Some analyses wereperformed to detect zinc on this type of electrode. Three differentconcentrations were detected by the standard addition method. Zincstandard solution (50 mg l⁻¹, 400 μl) was added to NH₄Ac (0.05 M, 75 ml)solution in three sequential steps giving zinc concentrations of 267,534 and 800 μg l⁻¹. A voltammetric scan was run after each addition. Thepeak heights for zinc were measured and corrected for offset, andpresented in FIG. 3. In FIG. 3 detection of zinc on silver with 4%bismuth alloy electrode is shown. Concentrations were 267, 534 and 800μg l⁻¹. All scans were performed in DPASV mode in NH₄Ac (0.05 M), with ascan rate of 15 mV/s, pulse height 70 mV and a deposition time of twominutes. Current values were corrected for offset. As a small frame upin the right corner in FIG. 3, is shown detection of zinc on the silverelectrode with 4% bismuth, but as current (μA) as a function ofpotential (mV).

[0038] 3.2 Silver Electrode with 4% PbO

[0039] The voltammetric scan was performed as described in section 2.Both deposition and start potential was −1200 mV. FIG. 4 shows a typicalplot of a blank NH₄Ac (0.05 M) solution and corresponding when zinc wasadded. As shown there is a high hydrogen over-voltage also on thiselectrode, as for the silver electrode containing 4% bismuth describedin section 3.1. In FIG. 4, detection of zinc on silver with 4% lead(II)oxid alloy lectrode is shown as a plot of current (μA) as a function ofpotential (mV). The concentration of zinc was 534 μg l⁻¹. All scans wereperformed in DPASV mode in NH₄Ac (0.05 M), with a scan rate of 15 mV/s,pulse height 70 mV and a deposition tim of two minutes.

[0040] When the zinc concentration exceeded 500 ppb, a change in thevoltammogram was detected around −1100 mV. This change may represent theformation of intermetallic compound. This phenomenon was not observed onthe silver electrode with bismuth in section 3.1.

[0041] Detection of cadmium was also impossible on this electrodebecause of the huge current level raising from ca. −600 mV, due to thegrate amount of lead inside the electrode.

[0042] 3.3 Silver Electrode with 4% Mercury

[0043] Analyses as in section 3.1 and 3.2 were performed for a silverelectrode containing ca. four percent mercury. FIG. 5 below shows theresult from zinc detection on this type of electrode. Four differentconcentrations were detected by the standard addition method. Zincstandard solution (50 mg l⁻¹, 150 μl) was added to NH₄Ac (0.05 M, 75 ml)solution in three sequential steps giving zinc concentrations of 100,200, 300 and 400 μg l⁻¹. A voltammetric scan was run after eachaddition. In FIG. 5 detection of zinc on silver with 4% mercury alloyelectrode is shown as a plot of current (μA) as a function ofconcentration (ppb). The concentration of zinc was 100, 200, 300 and 400μg l⁻¹. All scans were performed in DPASV mode in NH₄Ac (0.05 M), with ascan rate of 15 mV/s, pulse height 70 mV and a deposition time of twominutes. As a small frame to the right is shown a plot of current (μA)vs. potential (mV) for the main plot in FIG. 5.

[0044] A high degree of linearity was registered. This is indicated bythe relative high R² (R²=0,9724) coefficient between the experimentaldata end the applied linear fit in each case.

[0045] Also a detection of cadmium (200 ppb) was performed in a NH₄Ac(0.05 M) solution containing zinc (400 ppb), and are shown in FIG. 6. InFIG. 6 detection of zinc and cadmium on silver electrode containing 4%mercury is shown as a plot of the resulting current (μA) as a functionof potential (mV). Concentrations were 400 μg l⁻¹ zinc and 200 μg l⁻¹cadmium. The scan was performed in DPASV mod in NH₄Ac (0.05 M), with ascan rate of 15 mV/s, pulse height 70 mV and a deposition time of twominutes. The addition of cadmium had only an insignificant influence onthe peak height for zinc.

[0046] 4. Conclusions

[0047] Results in this present paper shows that electrode materialscomprising a metal or a compound with low hydrogen overvoltage changetheir hydrogen overvoltage properties substantially when contaminatedwith even small amounts of metals or compounds which show high hydrogenovervoltage. This extends greatly the range of potentially availablenon-toxic electrode systems and thereby analytical possibilities ofvoltammetry and related methods, also for online use in the field.

[0048] A very important point is that the counter electrode in athree-electrode system may be prepared with the same composition as theworking electrode. Then a new film may be plated prior to each new scanwith material from the counter electrode. With that a new workingelectrode surface may be generated continuously.

[0049] The specific composition of an electrode may be prepared in sucha way that it is optimal for some metals, particular considering thepossibility to prevent intermetallic compounds.

[0050] Having described preferred embodiments of the invention it willbe apparent to those skilled in the art that other embodimentsincorporating the concepts may be used. These and other examples of theinvention illustrated above are intended by way of example only and theactual scope of the invention is to be determined from the followingclaims.

REFERENCES

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1. Electrode for trace metal or trace compound detection in analytical voltammetry, characterized in that the electrode comprises a solid electrode material, wherein the electrode material is an alloy in a solid state, the alloy comprising a metal or compound having an overvoltage for hydrogen which is below a potential required for the meal or impound to be used alone for said detection, and less than 100% by weight of at least one second metal or compound giving the resulting to alloy an hydrogen overvoltage which is above the potential required for said detection.
 2. Electrode for trace metal or trace compound detection in electrochemical analysis, characterized in that the electrode comprises a solid electrode material, wherein the electrode material is an alloy in a solid state, the alloy comprising a metal or compound having a low hydrogen overvoltage, wherein the metal or compound is contaminated with a second metal or compound giving a high hydrogen overvoltage of the alloy.
 3. Electrode according to one of claims 1-2, characterized in that the metal or compound is metallic sliver, gold, copper, rhenium, rhodium, tungsten, iridium, molybdenum or platinum.
 4. Electrode according to one of claims 1-3, characterized in that the second metal or compound is mercury, bismuth, thallium, cadmium, lead or oxides of these metals.
 5. Electrode according to one of claims 2-4, characterized in that the alloy comprises less than 10% by weight of the second metal or compound.
 6. Electrode according to claim 5, characterized in that the alloy comprises 4% by weight of the second metal or compound.
 7. Use of the electrode according to claim 1 as a measuring electrode or other electrodes in an electroanalytical method, requiring high overvoltage for hydrogen.
 8. Use of the electrode according to claim 1 as a measuring electrode in voltammetric analysis.
 9. Use according to claim 8, characterized in that the voltammetric analysis is of the type differential pulse anodic stripping voltammetry. 